Conventionally, in land seismic surveys, an array of seismic sensors is positioned to detect acoustic signals reflected from earth formations. The seismic sensors may be either analogue geophones or digital accelerometers. The signals from these sensors are input to Field Units where, in the case of analogue geophones, the signal is converted to a high-precision digital sample stream, and where with either type of sensor the digital sample stream is transmitted in real-time over a communications network to a Central Unit to be recorded on bulk recording media. The communications network involved in this process may be a cable-based network with repeaters and battery feeds as required; it may be an entirely cable-free network utilizing wireless techniques to transfer the data; or the network may consist of elements of both cabled and wireless technologies.
A number of disadvantages have been identified with these conventional systems, which has led to the development of a number of land seismic acquisition systems which do not utilize a communications network to transfer the digital sample stream to a Central Unit for recording, but which instead record the data locally in the Field Unit in non-volatile memory. In the normal case, the Field Unit records the data locally for as long as its seismic sensors are required as part of the active sector of the survey. The Field Units are then transported to a Central Unit for connection to a transcription unit and subsequent uploading of the data from the Field Unit to the Central Unit.
The primary advantages proposed for this technique are:                i. Reduction in manpower requirement as no communications infrastructure needs to be deployed        ii. Increased productivity as acquisition is not delayed by faults in a communications network        
These advantages are mitigated, however, by a number of disadvantages which this invention seeks to address either wholly or in part. These disadvantages are:                i. The non-volatile memory within the Field Unit must be large enough to record ail the seismic data acquired while the Field Unit is active on a survey, which may be as long as 14 days in a normal survey, but in exceptional cases may be much longer and may be indeterminate.        ii. It is normally the case that the Field Unit must be transported to a data transcription system which will be used to transfer all the acquired seismic data to the central Data Recorder.        iii. The seismic data acquired during the survey will not be available for examination until all the Field Units have, been transcribed as described in ii above, which may be as much as 14 days after the start of the survey, involving substantial risk that poor quality data may be acquired before there is an opportunity to detect it.        iv. There is a risk that Field Units may malfunction, be stolen, or be misplaced during the survey involving the less of all the data acquired by them.        v. Substantial field crew effort is required to transport the Field Units to the transcription system in a timely manner, which impacts on the productivity of the survey.        vi. Unforeseen circumstances, such as bad weather conditions, may delay transportation of the Field Units to the transcription system, causing further delays to the processing of the data.        